The Universal Mobile Telecommunication System (UMTS) is one of the 3G mobile communication technologies designed to succeed GSM. 3GPP Long Term Evolution (LTE) is a project within the 3rd Generation Partnership Project (3GPP) to improve the UMTS standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, lowered costs etc. The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS system and evolved UTRAN (e-UTRAN) is the radio access network of an LTE system. As illustrated in FIG. 1, a radio access network typically comprises user equipments (UE) 150 wirelessly connected to radio base stations (RBS) 110a-c, commonly referred to as NodeB (NB) in UTRAN and eNodeB (eNB) in e-UTRAN.
Multimedia Broadcast and Multicast Services-Single Frequency Network (MBMS-SFN or MBSFN) is a broadcasting service that may be offered in cellular networks such as LTE, e.g. to support mobile TV. MBSFN offers an option to use an uplink channel for interaction between the service and the user, which is not a solution in usual broadcast networks. For example conventional digital television is only a one-way (unidirectional) system. SFN refers to that several transmitters simultaneously send the same signal over the same frequency channel. The aim of SFNs is efficient utilization of the radio spectrum, allowing a higher number of radio and TV programs in comparison to traditional multi-frequency network (MFN) transmission. An SFN may also increase the coverage area and decrease the outage probability in comparison to an MFN, since the total received signal strength may increase in positions midway between the transmitters.
In some situations, it is desired to have radio sites, such as RBSs in a cellular system, that alternate between a unicast service with transmission in multiple sectors and a broadcast service, such as MBSFN, using all antennas of the site concurrently for the transmission. In a first mode of operation, i.e. the dedicated unicast mode, the RBS covers several sectors or coverage areas with antennas pointing in different directions. This first mode is described with reference to FIGS. 2a-2c, showing an example of a site transmitting in unicast mode. In FIG. 2a three different unicast signals S1(t), S2(t) and S3(t) are transmitted over the three antennas 1, 2, 3 respectively, each signal using a dedicated power amplifier 20a-c. The three antennas 1, 2, 3 are pointing in different directions, as shown in FIG. 2b, and are thus covering different sectors or coverage areas as shown in FIG. 2c illustrating the beam pattern for each of the antennas 1, 2, and 3.
In a second mode of operation, also called the broadcast mode, a same signal S4(t) is broadcasted to users positioned in any direction around the site. With MBSFN for LTE, sites are synchronized and transmissions occur concurrently and on the same frequency resource from the different sites. The overall idea of broadcast services such as MBSFN, is to transmit an information carrying signal in all directions. With a site with antennas pointing in different directions, as the one schematically illustrated in FIG. 2b, a straightforward configuration used for MBSFN is illustrated in FIG. 3a. As coverage is vital for broadcasting, all existing PAs are used.
A problem with the above described second mode of operation in a configuration as the one illustrated in FIG. 3a and with the antennas configured as in FIG. 2b, is explained with reference to FIGS. 3b and 3c illustrating the resulting radiation diagram from all three antennas 1, 2, 3, together. The diagram in FIG. 3b and FIG. 3c corresponds to a configuration where d/λ=2 and d/λ=3 respectively, where the distance d is given in FIG. 2b, and λ is the wavelength of the carrier. The resulting beam pattern will contain a lot of deep nulls, i.e. very low radiated power in certain directions. This will e.g. be the case in the areas where the beam patterns from two of the antennas are overlapping. It should be noted that the radiation diagrams in FIGS. 3b and 3c, are illustrated with a linear scale. With a logarithmic scale the variations in signal amplitude would be less dramatic. The conclusion is that such a site, which in this example is a site with three PAs and three antennas, where the antennas are pointing in the directions 0°, 120° and 240°, will provide inadequate directional coverage when transmitting in broadcast mode. In the following, the array and antenna-beam pattern models that have been used to compute the resulting radiation pattern are disclosed. The array model used is:z1=d·cos(Θ1), Θ1=αz2=d·cos(Θ2), Θ2=α+π/3z3=d·cos(Θ3), Θ3=α−π/3where the parameters d and α are shown in FIG. 4a. zn (n=1, 2, 3) is the distance difference relative the origin of the diagram (or the centre of the array) for each of the antenna, in a certain direction given by the angle α under consideration. Furthermore, a simple beam pattern model for an antenna is assumed according to the following:
      g    ⁡          (      φ      )        ~            sin      ⁡              (                  v          ⁢                                          ⁢          φ                )                    v      ⁢                          ⁢      φ      where φ is the angle relative to the main direction of the antenna, and ν is a parameter that indicates how compact the beam from the antenna is. The parameter ν may vary between different types of antennas. The beam pattern, i.e. the antenna power in different angles, for different values of the parameter ν is illustrated in FIG. 4b. A real value of the antenna gain is assumed, which is not necessarily true in reality but may be used as a model to demonstrate the general idea.
Finally, the superposed electromagnetic field r(α) in the far field and in a certain angle α may be computed according to the following equation:
            r      ⁡              (        α        )              =                            g          ⁡                      (                          φ              1                        )                          ⁢                  ⅇ                      j            ⁢                                          2                ⁢                π                ⁢                                                                  ⁢                                  z                  1                                            λ                                          +                        g          ⁡                      (                          φ              2                        )                          ⁢                  ⅇ                                    -              j                        ⁢                                          2                ⁢                                                                  ⁢                π                ⁢                                                                  ⁢                                  z                  2                                            λ                                          +                        g          ⁡                      (                          φ              3                        )                          ⁢                  ⅇ                                    -              j                        ⁢                                          2                ⁢                π                ⁢                                                                  ⁢                                  z                  3                                            λ                                            where                    φ        1            =      α        ,                  φ        2            =                        rem          ⁡                      (                                                                                5                    ⁢                    π                                    3                                -                α                            ,                              2                ⁢                π                                      )                          -        π              ,                  φ        3            =                        rem          ⁡                      (                                                                                7                    ⁢                    π                                    3                                -                α                            ,                              2                ⁢                π                                      )                          -        π            